1. Field of the Disclosure
The present invention relates to methods, apparatuses and systems for reducing and eliminating toxic gases emitted from exhaust systems.
2. Description of the Related Art
The “background” description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description which may not otherwise qualify as prior art at the time of filing, are neither expressly or impliedly admitted as prior art against the present invention.
Exhaust gas or flue gas is emitted as result of the combustion of gaseous, liquid or solid fuels such as natural gas, gasoline or petrol, diesel fuel, fuel oil or coal. Depending on the type of combustor, exhaust gas is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle. Examples of the different types of combustion engines include internal combustion, spark-ignition, diesel, gas-turbine, jet, rocket and steam engines.
Exhaust gas is a major component of motor vehicle emissions and also emissions of other stationary industrial combustion engines. These emissions contribute to air pollution and are a major ingredient in the creation of smog in many large cities globally. A 2013 study conducted by Massachusetts Institute of Technology indicated that that 53,000 early deaths occur every year in the United States alone because of motor vehicle emissions. According to another study from the same institution, traffic fumes alone cause the death of 5,000 people every year in the United Kingdom.
The largest part of most exhaust or combustion gas is nitrogen, water vapor and carbon dioxide (except for fuels without carbon). These fluids are not toxic nor noxious, although carbon dioxide is generally recognized as a greenhouse gas that contributes to global warming. Additionally, a relatively small part of combustion gas is undesirable noxious or toxic substances, such as carbon monoxide from incomplete combustion, hydrocarbons from unburnt fuel (CxHy), nitrogen oxides (NOx) from excessive combustion temperatures, ozone (O3) and particulate matter (mostly soot). The U.S. Environmental Protection Agency estimates the average emission of a passenger car in the country and the results are as shown in Table 1 below.
TABLE 1Emission rate and annual emission ofan average passenger car in the U.S.ComponentEmission rate (g/km)Annual emission (kg)Hydrocarbons1.7535CO13.06261NOx0.8717.3CO22585190
To comply with the U.S. Environmental Protection Agency's stricter regulation of exhaust emissions, modern motor vehicles are each equipped with a catalytic converter which is a vehicle emissions control device that converts toxic pollutants in exhaust gas to less toxic pollutants by catalyzing a redox reaction (oxidation or reduction). Apart from motor vehicles, catalytic converters are also used in other internal combustion engines field by either gasoline/petrol or diesel, including lean burn engines. Catalytic converters are also used on electrical generators, forklifts, mining equipment, locomotives, airplanes and woodstoves, usually in response to government regulation, either through direct environmental or health and safety regulations.
A catalytic converter's construction includes three main components: catalyst, washcoat and substrate. The catalyst is a mixture of metals such as platinum, palladium, rhodium, cerium, iron, manganese and nickel. The washcoat is a carrier for the catalytic materials and is used to disperse the materials over a large surface. The substrate is a catalyst support and the core is usually a ceramic monolith with a honeycomb structure.
A two-way or oxidation catalytic converter performs two tasks simultaneously: oxidation of carbon monoxide to carbon dioxide and oxidation of unburnt and partially burnt hydrocarbons to carbon dioxide and water. Due to their inability to control nitrogen oxides, two-way catalytic converters are gradually superseded by three-way catalytic converters. Three-way catalytic converters have the additional ability to reduce nitrogen oxides to nitrogen and oxygen.
Although proven to be reliable and effective in reducing noxious tailpipe emissions, catalytic converters also have some shortcomings and limitations. For example, an engine equipped with a three-way catalytic converter must run at stoichiometric point, therefore increasing the consumption of fuel and adding approximately 10% more carbon dioxide emissions from the engine. Catalytic converters are designed to work within a very limited band of exhaust gas concentration. Gas concentrations that exceed or are below the limits will greatly reduce the efficiency and the life of a catalytic converter and it is very expensive to replace one.
The durability of a catalytic converter is also affected by thermal degradation and poisoning of the catalytic metals by impurities such as lead and sulfur in fuels and zinc, phosphorous and magnesium from lubricating oil additives.
Furthermore, unwanted reactions such as the formation of odoriferous hydrogen sulfide and ammonia can occur in three-way catalytic converters.
There have been many ongoing efforts that aim to overcome the shortcomings and limitations of the catalytic converter. However, the continuing prevalence of catalytic converters in motor vehicles in the market may infer that such efforts have not accomplished the desired success. Examples of these efforts in the prior art include U.S. Pat. No. 6,240,725, U.S. Pat. No. 8,601,800, U.S. Pat. No. 4,008,056, U.S. Pat. No. 5,857,324, JP2013/122225, KR1,335,260, CA2,301,347 and EP2,700,794 (each incorporated herein by reference in its entirety).
Accordingly, there is a need for new devices and systems that can replace, complement or improve the existing catalytic converters to reduce or eliminate the emission of pollutants from motor vehicle and industrial exhaust systems.